The
SI (Systeme Internationale)
system specifies a
base unit for each
type of measurement. This unit is a standard quantity for that property, and all
measurements of that property are
relative to that standard. Three of the base units are shown below.
metre (m) for length
3 m is 3 times the standard length. 10 m is 10 times the standard length.
kilograms (kg) for mass
second (s) for time
| prefix | symbol | factor × base unit | Used for quantities |
| kilo | k | 103 | of moderate size |
| deci | d | 10–1 |
| centi | c | 10–2 |
| milli | m | 10–3 |
| micro | µ | 10–6 | that are VERY small |
| nano | n | 10–9 |
| pico | p | 10–12 |
These
base units are related to
smaller or larger units by
factors that are powers of
10.
The k in kg is a prefix meaning that a kilogram is one-thousand (103) times larger than a unit of mass called the gram (g).
The kilogram was put in place as a standard before it was easy to measure smaller masses.
It has been retained as the standard because it is a large enough mass to measure very accurately.
Each
factor has a
symbol (like the k in kilogram).
Prefixes commonly used in chemistry are given in the table below along with their symbol and the factors by which they change the base unit.
Prefixes having
factors that are
positive powers of ten
make the unit larger (10
3 = 1000).
Prefixes having
factors that are
negative powers of ten
make the unit smaller (10
–3 = 1/1000 = 0.001).
The common very small units differ from one another by a factor of 10
3.